1. Field of the Invention
This invention relates to a rubber composition which is particularly suitable for making the treads of tires and the soles of shoes.
2. Description of the Prior Art
The treads of tires on the wheels of a vehicle have a by far smaller friction force on a frozen road surface than on a normal dry road surface. Spiked tires or tire chains are used to improve the friction of the treads on a frozen road surface and thereby the safety of the vehicle which is driven on such a surface. Both the spiked tires and the tire chains, however, damage the road surface when the vehicle is caused to make a sharp turn, or a sudden start or stop. The damaged road surface forms dust which is scattered by wind when the surface has dried. Moreover, the spikes or chains on the tires make noise as they hit the road surface when the vehicle is running.
A number of technical approaches have recently been made to improve the friction on ice of the rubber which is used for making the treads of tires. They are classified into two main groups of methods. One of the first group of methods comprises foaming rubber to form closed cells in it, as proposed in Japanese patent application laid open to the public under No. 89547/1988. This method gives rubber having a surface covered with a multiplicity of pores which exert a sucking effect on ice, exhibits a water-absorbing effect, and undergo a microscopic behavior causing the loss of energy, so that the rubber surface may produce a large amount of friction on ice. Studless tires formed from such rubber are commercially available.
Another attempt belonging to the first group that has been proposed comprises incorporating various kinds of materials into rubber, so that when the tires formed from such rubber and mounted on a vehicle are rotating, the incorporated material may drop from the tires and thereby form pores in the surfaces thereof. Sand, chaff, or other natural materials are, for example, incorporated into rubber. This method also gives a tread surface which exhibits a high degree of friction on ice.
The second group of methods comprises incorporating various kinds of materials having a high degree of hardness into rubber, so that the incorporated material may exert a scratching effect on ice to enable a tread surface to exhibit a high degree of friction on a frozen road, as proposed in Japanese patent publication No. 31732/1971, Japanese patent application laid open to the public under No. 147803/1976 and Japanese patent publication No. 52057/1981. These methods are definitely based on a mechanism which differs from that on which the first group of methods relies. As a matter of fact, it is often likely that the incorporation of a greater amount of any such material may give rubber exhibiting a higher degree of friction on ice.
It is, however, to be noted that not only friction, but also high wear resistance is essentially required of any rubber used to make a tread, or anything else that will be brought into frequent contact with a road surface. No rubber exhibiting a high degree of friction on ice is suitable for making a practically acceptable tread, if it is low in wear resistance. None of the known methods as hereinabove described can, however, produce rubber exhibiting both a high degree of friction on ice and a high degree of wear resistance. Rubber having both of the two properties is very difficult to obtain even by the incorporation of any hard material as hereinabove mentioned. It has hitherto been believed that the two properties as hereinabove stated cannot be attained at the same time.